THORLABS KT120 User manual

KT120(/M)
Fiber Launch System:
Free Space
User Guide

Fiber Launch System: Free Space
Table of Contents
Chapter 1 Warning Symbol Definitions.............................................................................................1
Chapter 2 Safety...................................................................................................................................2
Chapter 3 Recommended Set-Up ......................................................................................................3
3.1. Tools.............................................................................................................................. 3
Chapter 4 Operating Instructions ......................................................................................................4
4.1. Step 1: Preparing the KT120(/M).................................................................................. 4
4.1.1. Goals of this Step..................................................................................................................4
4.1.2. Procedure..............................................................................................................................4
4.2. Step 2: Rough Alignment of the Laser Beam Path on Your Optical Bench.............. 4
4.2.1. Goals of this Step..................................................................................................................4
4.2.2. Procedure..............................................................................................................................4
4.3. Placement of the KT120(/M) in the Laser Beam.......................................................... 4
4.3.1. Goals of this Step..................................................................................................................4
4.3.2. Procedure..............................................................................................................................5
4.3.3. Helpful Hint............................................................................................................................5
4.4. Step 4: Clamp the KT120(/M) in Place ......................................................................... 5
4.4.1. Goals of this Step..................................................................................................................5
4.4.2. Procedure..............................................................................................................................5
4.5. Step 5: Coarse Vertical Laser Alignment Through the KT120(/M)............................. 5
4.5.1. Goals of this Step..................................................................................................................5
4.5.2. Procedure..............................................................................................................................6
4.6. Step 6: Installation of the Coupling Optic ................................................................... 6
4.6.1. Goals of this Step..................................................................................................................6
4.6.2. Special Note ..........................................................................................................................6
4.6.3. Procedure..............................................................................................................................6
4.7. Step 7: Optical Power Measurement ........................................................................... 6
4.7.1. Goals of this Step..................................................................................................................6
4.7.2. Procedure..............................................................................................................................7
4.8. Step 8: Assembly of the Z Translator with the KT120(/M).......................................... 7
4.8.1. Goals of this Step..................................................................................................................7
4.8.2. Procedure..............................................................................................................................7
4.8.3. Special Note ..........................................................................................................................7
4.9. Step 9: Optimizing Coupling Efficiency ...................................................................... 8
4.9.1. Goals of this Step..................................................................................................................8
4.9.2. Procedure..............................................................................................................................8
4.9.3. Special Note ..........................................................................................................................8
4.10. Step 10: Final Optimiztion Using Mirror “B” and the XY Translator ......................... 8
4.10.1. Goals of this Step..................................................................................................................8
4.10.2. Procedure..............................................................................................................................8
Chapter 5 Regulatory ........................................................................................................................10
Chapter 6 Thorlabs Worldwide Contacts........................................................................................11

Fiber Launch System: Free Space Chapter 1: Warning Symbol Definitions
Rev A, July 30, 2021 Page 1
Chapter 1 Warning Symbol Definitions
Below is a list of warning symbols you may encounter in this manual or on your device.
Symbol Description
Direct Current
Alternating Current
Both Direct and Alternating Current
Earth Ground Terminal
Protective Conductor Terminal
Frame or Chassis Terminal
Equipotentiality
On (Supply)
Off (Supply)
In Position of a Bi-Stable Push Control
Out Position of a Bi-Stable Push Control
Caution: Risk of Electric Shock
Caution: Hot Surface
Caution: Risk of Danger
Warning: Laser Radiation
Caution: Spinning Blades May Cause Harm

Fiber Launch System: Free Space Chapter 2: Safety
Page 2TTN238232-D02
Chapter 2 Safety
WARNING
Never look directly into a laser. It is recommended that the laser power be limited to a few milliwatts
during the alignment process.
This “Operating Instruction” is written for visible laser beams only, if you are working with a IR laser it is
assumed that the reader has the appropriate IR viewers and the appropriate experience to work safely
with intense IR laser beams. If you have any questions please call Thorlabs for technical support.
Many fiber launching applications will require an optical isolator to prevent back reflection off the input end of the
fiber from reaching the laser. Consult your laser manufacturer to ensure that your laser will not suffer any
damage from a reflection of the laser beam back into your laser.

Fiber Launch System: Free Space Chapter 3: Recommended Set-Up
Rev A, July 30, 2021 Page 3
Chapter 3 Recommended Set-Up
It is required that the user place two adjustable mirrors between the laser and the fiber launch system for most
applications. (See Figure F) These mirrors will provide the adjustment necessary to align the laser beam with the
optical axis of the launch system. Placing the launch close to the second mirror will simplify the optimization of the
coupling.
The KT120(/M) has an optical axis that is 3.69” (85.6 mm for the KT120/M). This “Operating Instruction” assumes
that the height of your laser beam is from 3.57” to 3.81” above the surface of your optical bench. If you need
assistance resetting the height of your laser beam please call Thorlabs for technical support.
3.1. Tools
Target card marked with cross-hairs at 3.69” to aid in rough alignment (an index card or any stiff piece of
cardboard works well as a target).
Optical fiber with FC or SMA connector, sold separately, see your Thorlabs Catalog, fiber optic section.
Photo-diode package and oscilloscope (recommended) or optical power meter (See Step 9 for more information).
Thorlabs model DET110 photodiode package or equivalent is recommended.
Scotch tape - “magic” preferred over “clear”
Lens tissue

Fiber Launch System: Free Space Chapter 4: Operating Instructions
Page 4TTN238232-D02
Chapter 4 Operating Instructions
4.1. Step 1: Preparing the KT120(/M)
4.1.1. Goals of this Step
In this step the KT120(/M) will be partially disassembled to allow the laser beam to pass through the launch system.
This will provide an easy means for ensuring that the launch system’s optical axis is aligned with that of your laser
beam. The translators will be set in the middle of their ranges. The coupling optic and its extension tube will be
removed. The fiber optic cable, if attached, will be removed. At the end of this step the KT120(/M) launch system
will be placed out of the way until Step 3.
4.1.2. Procedure
The XY translator on the KT120(/M) needs to be adjusted so the movable center section (which is threaded) is
roughly centered in the body of the XY translator. Use the two adjusters on the XY translator; do not be too
concerned with getting the unit exactly centered, this will be accomplished in a later step.
Ensure that the optic is removed from the XY translator; it is easiest to remove both the optic and the stainless
steel extension tube as one piece. It may be easier to remove one of the extension rods that form the cage
around the lens to allow more room for you to grip the tube.
The position of the Z translator control needs to be set to the middle of its range.
If a fiber optic cable is attached to the KT120(/M) it needs to be removed and set aside until it is specifically called
for later in these instructions.
The KT120(/M) is now ready and should be placed off to the side until Step 3.
4.2. Step 2: Rough Alignment of the Laser Beam Path on Your Optical Bench
4.2.1. Goals of this Step
In this step the position of the laser beam with respect to the optical bench will be set. While a recommended
optical layout is shown in Figure “F”, this is only one of many configurations that will work. The important point is
that the laser beam be both parallel to and 3.69” (85.6 mm for the KT120/M) above your optical bench with at
least one mirror with two angular adjustments close to the input of the KT120(/M).
4.2.2. Procedure
With an optical layout similar to that of Figure “F”, ensure that the laser beam hits mirror A at a height of between
3.57” to 3.81”. Using the marked card mentioned in the above “Tools” section, ensure that the laser beam hits
mirror B at a height of 3.69”.
Move the marked card downstream of mirror “B” by about 18” (The further the better). Use mirror “B” to adjust the
height of the laser beam so that it is 3.69” above the optical bench. At this point it is assumed that the laser beam
is pointed in a direction that will allow the KT120(/M) to be inserted close (within a few inches) to a mirror mount.
4.3. Placement of the KT120(/M) in the Laser Beam
4.3.1. Goals of this Step
In this step the KT120(/M) will be set in place on the optical bench and aligned with the laser beam. By using the
iris diaphragm on the input end of the KT120(/M) and the aperture of the Z translator that accepts the fiber optic
on the output end the launch system will be aligned sufficiently to ensure at least some transmission through the
optical fiber when it is added to the system.

Fiber Launch System: Free Space Chapter 4: Operating Instructions
Rev A, July 30, 2021 Page 5
4.3.2. Procedure
Insert SM1FC or SM1SMA in the Z translator.
Place a piece of tape on the inside aperture of the FC/SMA receptacle that is located within the Z translator.
Close down on the iris diaphragm that is at the input end of the launch system.
Place the KT120(/M) in the laser beam with the iris as the input end. For ease of alignment later on it is
recommended that the KT120(/M) be positioned as close to the last mirror mount (Mirror “B”, in Figure F) as
possible.
Using Mirror “B”, adjust the vertical height of the laser to ensure that the beam is well centered vertically on the
input iris of the KT120(/M). Rotate the KT120(/M) to ensure that the laser beam is well centered horizontally on
the input iris of the KT120(/M).
Now, position the KT120(/M) so that the laser beam is centered simultaneously on the input iris and the aperture
of the FC/SMA receptacle. If the laser alignment is off on the FC/SMA receptacle at the output end in the vertical
plane (high/low) at this time do not be concerned. Step 5 will detail how to make vertical adjustments of the laser
beam.
The scotch tape is present to enhance the visibility of the laser beam through the back aperture of the KT120(/M).
For some applications it might be easier to remove the tape and view the transmitted spot.
4.3.3. Helpful Hint
A trick that often enhances use of a transmitted spots for alignment is to place a piece of lens cleaning tissue in
the laser beam just before the launch input and shake it around within the laser beam. This will scatter some of
the light out of the main portion of the laser beam, this scattered light will then illuminate the output aperture and
cast a halo around the main portion of the laser beam that is transmitted through the launch system. Both the halo
and the main portion of the laser beam should be visible on a card if the card is positioned close to the output end
of the launch system. The position of the main portion of the laser beam within the halo can be used to determine
if the laser beam is passing through the center of the rear aperture of the KT120(/M).
4.4. Step 4: Clamp the KT120(/M) in Place
4.4.1. Goals of this Step
This step requires that attaching of the KT120(/M) assembly to your optical bench. Special care needs to be taken
to ensure that the launch system does not shift as it is attached to your bench.
4.4.2. Procedure
Clamp the KT120(/M) launch system down to the optical bench using table clamps. We recommend three clamps
from our CL series table clamps. Check the alignment of the laser through the system to ensure that the
KT120(/M) did not move during clamping. Repeat step 3 if necessary.
4.5. Step 5: Coarse Vertical Laser Alignment Through the KT120(/M)
4.5.1. Goals of this Step
This step details the method for adjusting the vertical alignment of the laser beam. Because the KT120(/M) has a
fixed height any errors in the earlier steps where the beam height was set to 3.69” will result in a misalignment of
the laser beam vertically on the output aperture formed by the FC/SMA receptacle. Most of the horizontal
misalignment has been eliminated at this point by simply rotating the KT120(/M); however the same technique that
will be used to correct any vertical errors can also be used to fine tune the horizontal alignment. The two Mirror
Mounts “A” and “B” of Figure”F” will be used to adjust the vertical approach of the laser beam. Many of you will be

Fiber Launch System: Free Space Chapter 4: Operating Instructions
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familiar with the process of using two mirrors to align a laser beam through two apertures. The procedure given
below is detailed carefully for those who might lack this experience.
4.5.2. Procedure
Use Mirror “B” to carefully center the laser beam on the input iris of the KT120(/M) in both the horizontal and
vertical planes. Look at the position of the laser beam on the output aperture formed by the FC/SMA receptacle
and note the position of the laser beam. Is it high or low? Or is it to the right or left?
While looking at the spot on the input iris use Mirror “A” to move the laser beam a small amount in the same
direction as the error that is seen on output aperture of the KT120(/M). In other words if the laser beam is low on
the output aperture of the KT120(/M) then use Mirror “A” to drive the laser beam low on the input iris.
Now, using Mirror “B” adjust the laser beam so that it is once again centered on the input iris of the KT120(/M).
When you look at the output aperture you will note that the laser beam has moved in the direction opposite to the
original misalignment. The amount that the laser beam is corrected in each step of this process is a matter of
“feel” which will be learned quickly.
Continue this procedure until the laser beam is well aligned with both the input and output apertures of the
KT120(/M). Paying special attention to this alignment step will greatly simplify the whole coupling process later in
these instructions.
4.6. Step 6: Installation of the Coupling Optic
4.6.1. Goals of this Step
This step details the installation and centering of the coupling optic that focuses the laser beam down onto the
fiber optic. A target will be used to note the location of the laser beam after it passes through the KT120(/M). After
the target is in place, the optic will be added to the launch system. By ensuring that the laser beam hits the same
spot on the target with or without the lens in place, proper centering of the optic can be assured.
4.6.2. Special Note
The recommended optic for the KT120(/M) is our C230TM aspheric lens, we have found that this lens typically
gives the best results for visible and near IR lasers that have beam diameters around 1mm (0.04”).
4.6.3. Procedure
Remove the Z translator and at least one of the extension rods from the KT120(/M). If the tape is present that was
used to enhance the visibility of the laser on the FC receptacle remove it.
Place a business card or similar target about 5.90” (150 mm) from the output end of the KT120(/M) and mark the
location of the laser beam on the card.
Insert the lens in the stainless steel extension tube if it has been removed, and then assemble the extension tube
into the XY translator of the KT120(/M). Tighten securely in place.
Adjust the XY controls of the KT120(/M) until the expanded spot from the lens is centered in the same spot on the
target as the original spot (the spot before the optic was installed).
4.7. Step 7: Optical Power Measurement
4.7.1. Goals of this Step
This step documents the total power that is available at the input end of the fiber so that an estimate of the
coupling efficiency can be made.

Fiber Launch System: Free Space Chapter 4: Operating Instructions
Rev A, July 30, 2021 Page 7
4.7.2. Procedure
Using an optical power meter, for an absolute reading, note the total power just after focusing the optic. A
photodiode may also be used if just a relative measure of the power is enough for your application.
For safety reasons it is recommended that the power should be limited to less than 10mW during this entire setup
and alignment process. If your laser source is being attenuated during the alignment process you may want to
record both the attenuated power used during this setup and the laser power you will be using during normal
operation of your system.
It is strongly recommended that a photodiode sensor similar to the Thorlabs model DET110 be used in
conjunction with and oscilloscope to monitor laser at the output end of the fiber.
4.8. Step 8: Assembly of the Z Translator with the KT120(/M)
4.8.1. Goals of this Step
This step details the assembly and alignment of the Z translator that houses the input end of the connectorized
fiber optic cable with the KT120(/M) launch system. Using the fact that the end of the fiber is partially reflective the
back reflected spot will be used to set the coarse location of the Z translator with respect to the focused spot of
the laser beam
It is often helpful to dim room lights for this next set of steps.
4.8.2. Procedure
Attach the connectorized-input end of the fiber optic cable into the FC-SMA receptacle of the Z translator. Ensure
that the connector is firmly tightened.
In order to view any light coming out of the fiber optic cable, place the output end of the fiber up against a folded
business card so that the end of the ferrule almost touches the card.
Close the input iris of the KT120(/M) to approximately 70% of the input laser beam diameter.
Slowly step the Z translator toward the focus of the laser beam while monitoring the back reflected spot that is
visible on the back side of the iris. The back reflected spot will change with the position of the Z translator. Slide
the Z translator through the focus of the laser beam to familiarize yourself with the spot on the back of the iris. The
goal of the next paragraph will be to set the position of the Z translator at the point at which the back reflected
spot is roughly the same size as the opening in the iris. This is achieved when the fiber end is at the focus of the
laser beam.
After you become familiar with the reflected spot that is viewed on the iris, slide the Z translator back away from
the optic a millimeter or so past the focus of the laser beam. This will position the Z translator for the next step as
well as ensure that a small portion of the laser beam will couple through the fiber optic. Verify that some light is
visible at the output of the fiber. If no light is visible move the Z translator toward the focus in small steps.
Now the Z translator can be moved toward it’s final location in a step wise fashion. After each step use the XY
controls of the KT120(/M) to peak the transmission through the fiber. This will ensure that some power will always
be visible at the output end of the fiber as the fiber approaches the focused spot of the laser beam. Continue the
process until the back reflected spot is approximately the same size as the opening in the iris.
4.8.3. Special Note
The transmission through the KT120(/M) becomes very sensitive as the fiber end approaches the focus of the
laser beam. This is because the laser spot on the end of the fiber is going from a very large spot that over fills the
fiber to a spot that is only a few microns in diameter. Once the system becomes very sensitive to touch you are
probably within the range of the Z translator and can therefore lock the Z translator in place and proceed with the
next step.

Fiber Launch System: Free Space Chapter 4: Operating Instructions
Page 8TTN238232-D02
Lock the four set screws on the Z translator, and then use the XY controls on the KT120(/M) to peak the
transmission through the fiber.
4.9. Step 9: Optimizing Coupling Efficiency
4.9.1. Goals of this Step
This step details the optimization of the transmission through the fiber by walking the Z translator toward the focus
of the laser beam while tweaking the XY controls. We strongly recommend using a photodiode sensor and an
oscilloscope for this process over an Optical Power Meter. This is because the photodiode when used with an
oscilloscope will allow you to better “see” where the peak in transmission is as you move one of the KT120(/M)
controls. Most Optical Power Meters have response times that are to slow to give a good sense of the peak
transmission as a control is moved.
4.9.2. Procedure
Peak the transmission through the fiber using the X and Y adjusters. Note the peak transmission.
Chose a direction and step the Z translator (see the note below on the step size), peak the transmission using the
XY controls, note the peak transmission. If the power drops due to the stepping of the Z translator reverse the
direction and try again. Repeat this process, tweaking the XY translator after each step in the Z direction until no
further gains in transmission are achieved.
4.9.3. Special Note
Step Size, the step size will need to be changed as the system approaches optimization. A good “rule-of-thumb”
is to step the Z translator so that the transmission drop by about 20%. This will result in fairly large steps when the
input of the fiber is a large way from the fiber. This will work well until you get the fiber end very close to the focus
of the laser beam, at this point you will need to use your best judgment in order to optimize the transmission.
4.10. Step 10: Final Optimiztion Using Mirror “B” and the XY Translator
4.10.1. Goals of this Step
This step details the final optimization of the KT120(/M) launch system. Mirror “B” will be used to optimize the
coupling into the fiber by adjusting the angle of the incoming laser beam. This will be accomplished by stepping
the angular controls of mirror “B” while tweaking the transmission using the XY controls. First, one angular control
will be chosen to be stepped in one direction. The XY controls of the KT120(/M) will then be used to maximize the
efficiency of the system. If the power increases, this process will be repeated, if it drops, the process will be
reversed. This process will be repeated on both angular controls of mirror “B” until no further power increases are
obtained.
4.10.2. Procedure
Peak the XY controls then tweak the Z translator; note the transmission through the fiber. Chose one of the
angular controls on mirror “B” and step it in one direction until the transmitted power through the fiber drops by
about 5%.Tweak the transmission through the system using the XY controls. Repeat this process; reversing the
direction of the angular step if the power drops, until no increases in power are detected.
Repeat the last step on the other angular control on Mirror “B”.
Afinal round of peaking on the XY translator and the Z translator will finish the optimization process.
As a point of reference, the KT120(/M) has been tested in our laboratory with a HeNe laser and our single mode
fiber FS-SN-3224. An efficiency of 83% is routinely achieved. If you are using a laser diode source an efficiency of
35-40% is typical.

Fiber Launch System: Free Space Chapter 4: Operating Instructions
Rev A, July 30, 2021 Page 9
Figure F The recommended setup for the KT120(/M). Two mirrors are ideal for aligning the laser beam
with the optical axis of the KT120(/M) Fiber Launch System.

Fiber Launch System: Free Space Chapter 5: Regulatory
Page 10 TTN238232-D02
Chapter 5 Regulatory
As required by the WEEE (Waste Electrical and Electronic Equipment Directive) of the European Community and
the corresponding national laws, Thorlabs offers all end users in the EC the possibility to return “end of life” units
without incurring disposal charges.
•This offer is valid for Thorlabs electrical and electronic equipment:
•Sold after August 13, 2005
•Marked correspondingly with the crossed out “wheelie bin” logo (see right)
•Sold to a company or institute within the EC
•Currently owned by a company or institute within the EC
•Still complete, not disassembled and not contaminated
As the WEEE directive applies to self-contained operational electrical and electronic products, this end of life take
back service does not refer to other Thorlabs products, such as:
•Pure OEM products, that means assemblies to be built into a unit by the user (e. g. OEM laser driver cards)
•Components
•Mechanics and optics
•Left over parts of units disassembled by the user (PCB’s, housings etc.).
If you wish to return a Thorlabs unit for waste recovery, please contact Thorlabs or your nearest dealer for further
information.
Waste Treatment is Your Own Responsibility
If you do not return an “end of life” unit to Thorlabs, you must hand it to a company specialized in waste recovery.
Do not dispose of the unit in a litter bin or at a public waste disposal site.
Ecological Background
It is well known that WEEE pollutes the environment by releasing toxic products during decomposition. The aim of
the European RoHS directive is to reduce the content of toxic substances in electronic products in the future.
The intent of the WEEE directive is to enforce the recycling of WEEE. A controlled recycling of end of life products
will thereby avoid negative impacts on the environment.
Wheelie Bin Logo

Fiber Launch System: Free Space Chapter 6: Thorlabs Worldwide Contacts
Rev A, July 30, 2021 Page 11
Chapter 6 Thorlabs Worldwide Contacts
For technical support or sales inquiries, please visit us at www.thorlabs.com/contact for our most up-to-date
contact information.
USA, Canada, and South America
Thorlabs, Inc.
[e-mail]
[e-mail]
UK and Ireland
Thorlabs Ltd.
[e-mail]
[e-mail]
Europe
Thorlabs GmbH
[e-mail]
Scandinavia
Thorlabs Sweden AB
[e-mail]
France
Thorlabs SAS
[e-mail]
Brazil
Thorlabs Vendas de Fotônicos Ltda.
[e-mail]
Japan
Thorlabs Japan, Inc.
[e-mail]
China
Thorlabs China
[e-mail]

www.thorlabs.com